In recent years, there has been proposed and commercialized an X-ray imaging apparatus which uses a solid-state image pickup device, such as CCD or amorphous silicon semiconductor. The X-ray imaging apparatus is a system which directly digitalizes X-ray images of a patient with a plurality of photoelectric conversion elements as imaging devices arranged in two-dimensional array to read the digitalized images through a scintillator (a visible light converting phosphor), as is the case with a conventional film system. The X-ray imaging apparatus, providing digital images almost in real time, has more significant advantages than the above-described film system or a photostimulable phosphor system. Especially, since the amorphous silicon can be prepared with a large area, an X-ray imaging apparatus with such amorphous silicon can image even a large site as seen in chest radiographing at the same magnification. Accordingly, the X-ray imaging apparatus, having high utilization efficiency of light, is expected to provide a high S/N ratio.
However, in producing many photoelectric conversion elements having large areas using amorphous silicon thin-film, a trace quantity of impurities mixed during a production process or an increase in the quantity of dangling bond causes a crystal lattice defect level in the film, which acts as a trapping level and results in unnecessary dark currents in a photoelectric conversion process to decrease an S/N ratio. To drive a photoelectric conversion substrate for relieving these dark currents, there is proposed a method for waiting for several to a few tens of seconds after photoelectric conversion elements (or switching devices) are biased, so as to relieve dark currents, and thereafter conducting photoelectric conversion. However, application of this method to an X-ray imaging apparatus causes the following problem: a cycle of radiographing a plurality of patients is increased or the operability of the apparatus is degraded.
Accordingly, to solve such a problem, there has been proposed a radiation imaging apparatus such as a apparatus disclosed in Japanese Patent Application Laid-Open No. 2004-33659 or Japanese Patent Application Laid-Open No. H02-22590.
The radiation imaging apparatus is provided with a light source including of a combination of an LED (light emitting diode) and a light guide plate on a photoelectric conversion substrate and emitting a light in a surface manner or a sheet-shaped light source using an EL (electroluminescence) device. The light source promotes charge injection into the trapping level of a photoelectric conversion element by emitting the light transmitted by the light source to the photoelectric conversion element, and is used for an optical calibration operation improving the characteristics of the photoelectric conversion element. Furthermore, Japanese Patent Application Laid-Open No. H10-186045 discloses a radiation detection apparatus which can detect optical I/O characteristics of a photoelectric transducer and a phosphor without need of irradiating radiation and correct radiation images using the detection results.